The present invention relates to an arrangement for effecting corrections of time base errors present in reproduced and/or transmitted audio information of the analog type.
For purposes of explanation, reference will be made to errors of the sort which are associated with reproductions from mechanical recording devices. Typically, such errors are in the form of speed, flutter and multichannel delay errors.
An example of speed error would be the playback of a magnetic tape at a speed slightly faster or slower than the speed at which it was recorded. This could be the result of rotational speed error of the capstan motor, incorrect capstan diameter (due to wear, for example), or slippage between the magnetic tape and the capstan due to an imbalance of tape tension. Speed error results in a change of pitch in a recording as it is played back. A speed error of as little as 0.1% can result in a change of pitch detectable to a trained musician. A speed error of 1% results in a noticeable pitch change. Speed error also causes a recording to play back over a different period of time. For example, a recording that was made over a period of 60 minutes would play back over 59 minutes and 24 seconds, in the case of a 1% error.
Flutter is a form of speed error and is an alternatingly increase and decrease in instantaneous speed at a cyclic rate, typically between 2 and 10 Hz. Flutter results from mechanism imperfections of all recording devices. A capstan that is not perfectly round (elliptical) will cause flutter in a magnetic tape recorder. Mechanical "play" in bearings and drive motors with non-uniform torque are other examples of sources of flutter. The primary objection to flutter is its audible effect which, in severe cases, sounds like a "fluttering" or vibrato sound. Flutter is measured by recording a pure tone and then measuring the frequency modulated component of the tone upon playback. A weighting curve is usually applied which takes into account the psychoacoustic effect of the flutter rate and the result is expressed in a percentage. Typical flutter measurements for professional tape machines vary from about 0.03% to 0.15%.
Delay in error is a problem associated with recording devices (normally magnetic tape recorders) which have more than one audio channel or "track". Mechanical imperfections, such as an error in tape head azimuth, can cause the playback of one or more channels to lead or lag a reference channel in time. Tape skew is another potential source of this error. Delay error is seldom more than 300 microseconds and usually is of no consequence until two or more tracks are electrically mixed. If both tracks contain components of the same audio signal, a nulling of this signal will result at a frequency whose period is equal to twice the delay error and at integral multiples of this frequency. For example, assume that a stereophonic (two track) tape machine has a delay error of 50 .mu.s. If the outputs of this machine are electrically mixed to form a monaural (one channel) signal, elements of the audio which are present on both the original two tracks at about the same level will cancel each other in the vicinity of 10 kHz. This is because 50 .mu.s corresponds to 180.degree. of phase at 10 kHz. This effect is particularly troublesome in the field of FM broadcasting. In this case, a significant number of listeners are listening to stereo broadcasts on monaural receivers and are hearing the electrical sum of both stereo channels. Delay error is not audible to the stereo listener but the monaural listener hears a very degraded high frequency response of the center channel components of the audio.
In the past, efforts have been made to minimize time base errors in audio equipment. An example is the arrangement for automatic alignment of the record head azimuth for least delay error for a magnetic tape recording and reproducing machine which is disclosed in U.S. Pat. No. 4,101,937, issued to John P. Jenkins on July 18, 1978. This known arrangement employs audible tones which must be erased before audio is recorded on the cartridge. Such a system corrects only for delay error contributed by the record head and only as that error is measured by the playback head of that same machine. It is not a real time continuous correction system which corrects for any error, regardless of source, each time the tape is played.